This invention relates to a time-delay-triggered TRAPATT oscillator circuit comprising a TRAPATT diode, a length of delay line connected in series with the diode and a matching filter in the form of a chain of sections of transmission line of successively different impedances connected to the end of the delay line remote from the diode. Such a circuit is described by W. J. Evans in "Circuits for high-efficiency avalanche-diode oscillators", I.E.E.E. Trans. MTT-17, 1060, (1969). However, it has been reported (J. E. Carroll, "The use of pseudo-transients in the solution of the Evans TRAPATT circuit", Proceedings of the 8th International MOGA Conference, Amsterdam, 1970; J. E. Carroll and R. H. Crede, "A computer simulation of TRAPATT circuits", Int. J. Electron. 32, 273 (1972)) that successful operation of this circuit depends markedly on a number of parameters, and the complexity of the circuit (which comprises series tuners in the form of transmission lines of fairly critical lengths and impedances) is a considerable hindrance to the design and construction of simple TRAPATT oscillator modules. In particular, since the Evans circuit relies for its operation upon repeated triggering of the diode by pulses reflected from the junction of the delay line and the filter, then reflections from within the filter itself or from circuit elements beyond the filter can cause unwanted, spurious triggering. As can be appreciated from FIG. 6 of the Evans paper, such reflections from within the filter can be caused by large impedance mismatches between the successive portions of the filter which, in the arrangement described with reference to FIGS. 4 to 6 of the Evans paper, is in the form of a co-axial line with tuning sleeves.
It has been observed in practice that if the matching filter is terminated in a component, such as a further filter or circulator, which on a time-domain reflectometer has a response similar to that of the matching filter, then coherence of the TRAPATT oscillation can be greatly reduced. It is thought that this degraded performance is due to spurious trigger voltages caused by reflections from the terminating component.